The ability of airborne leak detection technologies to detect hydrocarbon leaks from gas pipelines depends greatly on environmental conditions of the ground and atmospheric zones in the leak vicinity as well as on the flow rate of the leak to be detected. The soil type will determine conditions such as porosity, bulk density, water content and existence of vertical dispersion-paths that determine how quickly the gas will be able to migrate to the ground surface, as well as what the flux profile will be at the surface. The leak flow rate will determine how quickly the gas will be pushed through the dispersion-paths in the soil, and the combination of these two parameters will determine the main flow mechanism experienced by the leak. If the flow is primarily advective as determined by the Peclet number, a plug-flow model can be adopted by creating an iterative model to solve for the theoretical driving source pressure. However, if the leak rate is small, the flow is considered to be primarily diffusive, and numerical methods can be used to solve for the timedependent concentration and flux profiles at the ground surface. The resulting flux profiles from either the advective or diffusive models can then be input into an air-dispersion model to predict the resulting atmospheric methane concentrations that could be measured by airborne leak detection methods. The atmospheric concentrations are highly dependent on flux profile development times at ground level as well as on meteorological conditions. A 0.283 m3/hr (10 scfh) leak rate originating at 0.7 m below ground surface can be estimated to reach a steady-state plume after approximately nine hours for a 5 m/s wind speed and a slightly unstable atmospheric stability class. Under the same leak depth and meteorological conditions, a 28.3 m3/hr (1,000 scfh) leak is estimated to reach its steady state value after approximately three hours of leakage. The resulting gas concentrations in the atmosphere will depend on the individual leak site parameters. The models developed here allow for various combinations of leak rates, soil conditions, and atmospheric conditions to be modeled to assess the 'wait' time required before launching the leak detecting aircraft, as well as to predict the minimum leak rate detectable by a given technology.
Prediction of gas transport through ground and atmosphere to determine the ability of airborne leak detection methods to detect pin-hole leaks from buried gas pipelines
Vorhersage des Gastransports durch Boden und Atmosphäre zur Ermittlung der Fähigkeiten luftgestützter Leckageerkennungsverfahren bei der Detektion sehr kleiner Leckagen bei unterirdischen Gasleitungen
2009
18 Seiten, 16 Bilder, 35 Quellen
Conference paper
English
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